1. Field of the Invention:
The present invention relates to a composite material having high mechanical strength and heat resistance which is suitable for use as automotive parts, aircraft parts, and building material. More particularly, it relates to a composite material composed of a resin composition and a layered silicate which are bonded to each other through ionic bond and are uniformly mixed with each other, and also to a process for producing the composite material.
b 2. Description of the Related Art:
Various attempts have been made so far to incorporate an organic polymeric material with an inorganic material such as calcium carbonate, clay mineral, and mica for the improvement of its mechanical properties. Unfortunately, the incorporation of an inorganic material results in a brittle composite material because of the very poor bond strength between the inorganic material and the organic polymer matrix. In addition, the amount of an inorganic material that can be incorporated is limited. There is a known technique to increase the bond strength between an inorganic material and a polymer material. It is the treatment of an inorganic material with a silane coupling agent. However, the improvement achieved by this method results from van der Waals' forces between an inorganic material and an organic polymeric material, and the treatment merely improves the affinity of the two components and is not enough to add reinforcement and increase heat resistance.
For the purpose of improvement of mechanical characteristics, vulcanized rubber is incorporated with a large amount of reinforcing agents such as carbon black and inorganic fillers. Carbon black, among others, produces a remarkable reinforcing effect on vulcanized rubber because it has a variety of active functional groups such as carboxyl group, carbonyl group, phenolic hydroxyl group, and quinone group on the surface of the particles. The reinforcing effect results from the strong mutual action between the functional groups and the polymer chains constituting the rubbery polymer which is the matrix of vulcanized rubber. (See J. B. Donnet and A. Voet, "Carbon Black" (1976), Marcel Dekker.)
For reasons mentioned above, carbon black is still a major reinforcing agent for vulcanized rubber; however, it has a disadvantage that it is produced from petroleum or natural gas whose supply is limited and that it raises the viscosity of the compound and impairs the processability of the compound when it is incorporated in a large amount into rubber. To solve this problem (poor processability of carbon black-filled system), there has been proposed the grafting of polymer chains onto the surface of carbon black particles. However, this new technique is still under development.
On the other hand, inorganic fillers come in various forms, such as fiber, needle, plate, and granules. When it comes to reinforcing ability, fillers of platy form or needle form are advantageous over spherical carbon black.
With the foregoing in mind, the present applicant previously filed "Resin Composition Containing Polyamide" (Japanese Patent Laid-open No. 83551/1982) which comprises a polyamide resin and flaky vermiculite dispersed therein having an average aspect ratio of 5 or over. This resin composition is characterized by that the resin is incorporated with flaky vermiculite having a high aspect ratio (size-to-thickness ratio of particle), thereby to increase the mechanical strength of the resin (organic polymeric material). This resin composition certainly has an improved mechanical strength over conventional resin compositions; however, the improvement is offset by some drawbacks. That is, the flaky vermiculite does not have a sufficiently high aspect ratio because it is produced by mechanical crushing. Moreover, the flaky vermiculite ha to be added in a large amount to produce necessary strength, because of the poor bond strength between the vermiculite layers and the matrix. This might invite the danger of making the resin composition brittle.
In the meantime, several attempts have been made to obtain a composite material by synthesizing a polymer such as polyamide and polystyrene in the space between layers of clay mineral. A disadvantage of the conventional technique is that the molecular chains of the organic polymer do not infiltrate sufficiently into the interlayer space of the clay mineral and consequently the layers of silicate are not uniformly dispersed in the organic polymer. For the uniform dispersion, it is necessary to reduce the aspect ratio of the stratiform mineral, and this adversely affects the mechanical strength of the composite material. An additional disadvantage is that the bonding between the polymer material (matrix) and the layered mineral is not sufficient to provide the desired reinforcing effect.
The combination of a layered silicate and a polymeric substance is used to produce a composite material or to improve the storage stability of epoxy resin. The composite material is composed of polyamide and layered silicate dispersed therein in the form of molecule, because the polymerization of polyamide starts in the interlayer space of the layered silicate. The polymerization is induced by the catalytic effect of organic ions including ##STR1## present in the interlayer space of the layered silicate. (See Japanese Patent Laid-Open Nos. 72723/1987, and 74957/1987 (which corresponds to U.S. application No. 909,472).) The improvement of the storage stability of epoxy resin is based on the idea that the layered silicate which has teken up a hardener releases the hardener slowly, thereby bringing about the cross-linking reaction. ("Epohard 3000," a product of Adachi Shinsangyo Co., Ltd.)
The above-mentioned composite material containing polyamide resin as a polymeric substance has a disadvantage that it has an increased water absorption and a decreased dimensional stability. Moreover, the composite material in wet state decreases in glass transition point by 40.degree. C. or more as compared with that in dry state. In addition, the composite material decreases in resistance to chemicals such as acids and calcium chloride.
There are several known composite materials formed by the combination of rubber and silicate (clay mineral including hard clay, soft clay, and talc). However, no attempt has been made to combine rubber with an inorganic filler by utilizing the reaction in the interlayer space of a layered silicate, thereby greatly improving the mechanical properties of rubber. Being hydrophilic, a layered silicate is poor in affinity for rubber and also poor in dispersion into rubber. Thus, there has been a demand for a rubber compound in which a layered silicate is uniformly dispersed.